Sixty male volunteers were randomised to take 10-16 ml of a fish oil supplement (MaxEPA) or 10-16 ml of olive oil for a period of 3-6 weeks. A fall in serum triglyceride of 54% (P less than 0.01) and a fall in diastolic blood pressure of 7% (P less than 0.05) was attributable to taking fish oil supplements. The bleeding time was prolonged by 12%, but this did not reach conventional levels of statistical significance. A global test of heparin-neutralising activity, the heparin thrombin clotting time, increased by 14% (P = 0.05) but there was no demonstrable effect on thrombin time, fibrinogen or (intraplatelet) platelet factor 4. A fall in red cell pore transit time of 23% was attributable to fish oil, but was not statistically significant. There was no convincing evidence of an effect of fish oil supplementation on total serum cholesterol, HDL-cholesterol, blood counts or platelet aggregation. A beneficial effect of fish oil on the cardiovascular risk profile was confirmed in this study. However, with this regime changes in total cholesterol, HDL-cholesterol and platelet aggregation are of unlikely clinical importance.